The present invention relates to a process for manufacturing paper and more particularly to adding to a dilute paper pulp slurry prior to sheet formation a modified starch dispersion of a cationic starch which has been gelatinized and reacted with a blocked glyoxal resin.
Industrial starch may be utilized in a wide variety of applications including as coatings for paper or paper board and as a bonding wet end additive in papermaking. Starch compositions may desirably be prepared in the form of aqueous dispersions capable of being added to the pulp slurry.
For many commercial applications starch is gelatinized by the end user prior to being used. Gelatinization occurs after starch granules are dispersed as a slurry in water with the resultant aqueous slurry being heated to over 50.degree. C. and usually over about 65.degree. C. Under such conditions starch grains tend to absorb water, swell, and eventually rupture to allow starch fragments and molecules to disperse in water. This rupturing and dispersion is generally referred to as "gelatinization" and is an irreversible reaction resulting in a relatively thick starch dispersion.
The cross-linking of starches with multi functional reagents which are reactive with starch hydroxyl groups is well known. Glyoxals and polyaldehyde compounds and resins have been previously utilized as cross-inkers. The simple mixing of glyoxal with a starch dispersion will provide a gel. U.S. Pat. No. 4,455,416 describes a paper coating containing starch binder and a cyclic urea/glyoxal/polyol condensate as an insolubilizer for the binder. As an insolubilizer the glyoxal condensate is inactive until the coating is applied and cured upon drying whereupon the glyoxal crosslinks the starch to impart water resistance. U.S. Pat. No. 4,021,260 describes ethoxylated fatty alcohols as starch viscosity control agents. U.S. Pat. No. 3,324,057 discloses the use of dialdehyde starch in the preparation of paper coatings. U.S. Pat. No. 3,740,391 describes the production of aqueous dispersions of a starch first reacted with an acrylamide and, subsequently, with glyoxal. U.S. Pat. No. 4,013,629 discloses a glyoxal binder system. U.S. Pat. No. 4,425,452 discloses coating paper material via an enzymatically converted starch. British Patent No. 2017124 discloses polysaccharides cross-linked with glyoxal.
A stable fluid aqueous modified starch dispersion is prepared in U.S. Pat. No. 5,032,683, the disclosure of this patent being incorporated herein by reference. This starch dispersion is prepared by gelatinizing an aqueous slurry of a starch and reacting with a glyoxal compound. The present invention has found that a cationic starch dispersion modified with blocked glyoxal resin when added in the wet end of a papermaking process provides significant improvements in strength to the resultant paper.